Si, si, or SI may refer to (all SI unless otherwise stated):

• International System of Units (Système international d'unités), the modern international standard version of the metric system
• Signal integrity, electronic circuit tools and techniques that ensure electrical signals are of sufficient quality for proper operation
• Si, the chemical symbol for silicon
• Si(x), the sine integral in mathematics
• Survey of India, an Indian government agency responsible for managing geographical information about India
• Simple Interest (Mathematics)

Sì (pronounced , or ) is traditional spherical dessert made by glutinous rice, eaten for celebration for the Winter Solstice festival in Fuzhou, China. It is usually without stuffing. The sì making is to powder glutinous rice, and press the powder until almost dry, and then roll the dry powder into spherical shape, and finally add fried (even burnt) soybean powder mixed with brown sugar onto the glutinous sphere.

.si is the Internet country code top-level domain (ccTLD) for Slovenia. It is administered by the ARNES, the Academic and Research Network of Slovenia. In 2010, the registry has hosted 80,000 domain names. In 2012, that number increased to 100,000 .si domain names when the domain zj.si was registered by Jasico d.o.o.

Cordyline fruticosa is an evergreen flowering plant in the Asparagus family, Asparagaceae, known by a wide variety of common names, including cabbage palm, good luck plant, palm lily, ti plant, Ki, La‘i (Hawaiian), Ti Pore (Maori), Si (Tongan), Lauti (Samoan), and Auti (Tahitian).

The International System of Units (, SI) is the modern form of the metric system, and is the most widely used system of measurement. It comprises a coherent system of units of measurement built on seven base units. It defines twenty-two named units, and includes many more unnamed coherent derived units. The system also establishes a set of twenty prefixes to the unit names and unit symbols that may be used when specifying multiples and fractions of the units.

SÍ is an abbreviation for Sørvágs Ítóttarfelag (Sørvágur Sport Association), a sports association in the Faroe Islands. It was founded March 17, 1905.